The present invention relates to a process for producing styrene and acrylonitrile based copolymers. More particularly, the present invention relates to a process for producing copolymers of styrene and acrylonitrile (SAN) and copolymers of styrene and acrylonitrile containing an elastomer (ABS).
By comparison with homopolymers of styrene, SAN resins offer many advantages, such as a lower thermal deformation, better aging, high gloss and improved mechanical properties. This improvement in several properties becomes apparent when the acrylonitrile content in SAN resins is as low as 5% of weight. However, SAN resins usually are prepared with an acrylonitrile content varying between 25 and 90% by weight.
By polymerizing vinyl compounds, more particularly a mixture of styrene and acrylonitrile, in the presence of an elastomer, particularly a polybutadiene rubber, ABS compositions are obtained which consist of said rubber and acrylonitrilestyrene copolymers, a part of said copolymers being grafted on the rubber particles. Such ABS resins have good mechanical properties, particularly high-impact resistance, when the rubber is grafted by at least 5% of its own weight of styreneacrylonitrile copolymers, the remainder of these copolymers forming a continuous phase wherein the particles of grafted rubber are dispersed.
For preparing ABS or SAN resins, usual polymerization processes may be used, either mass, suspension or emulsion polymerization. In commercial practice, however, a two-step copolymerization is most often used, such two-step process comprising a mass precopolymerization step followed by a suspension copolymerization step. The resins so obtained are beads which are easily separated from the reaction mixture and may then be washed and dried. However, when using any one of these processes, the ratio of styrene to acrylonitrile in the copolymers is always lower than the ratio of styrene to acrylonitrile in the starting monomers mixture if the acrylonitrile content in said monomer mixture is higher than 25%. Consequently, in such case, residual acrylonitrile remains in the resin when the copolymerization is not completed at 100%. It is not advisable, however, to carry out the copolymerization to the very end with a view of avoiding the presence of residual acrylonitrile in the copolymer. The reaction rate is lower and lower when the monomer concentration decreases. Further, by carrying on the copolymerization when the total amount of styrene is already copolymerized results in production of the homopolymer of acrylonitrile and this homopolymer has a detrimental effect on the color of the ABS resins. The presence of residual acrylonitrile (which is very toxic) in the resins is harmful when these resins are handled and worked. Further, acrylonitrile is water soluble and the residual process water contains acrylonitrile which has to be removed. For instance, ABS resins which are manufactured from a mixture of styrene and acrylonitrile with an acrylonitrile content of 29% may contain 1 to 1.5% of residual acrylonitrile and the waste process water may contain about 0.5% of acrylonitrile when the content of residual styrene is 0.5%.
The object of the present invention is to provide a new and improved process for producing ABS and SAN resins which are practically free from residual acrylonitrile.